1. Technical Field
The present invention relates to a transport-object transporting device that switches a rotating direction of a discharge roller between a forward direction and a reverse direction to turn over a transport object, such as a sheet, transported with its first face facing one direction by a transport roller, so as to transport the transport object with its second face facing the one direction. Specifically, the invention relates to a transport-object transporting device that can switch the discharge roller between a nip position and a release position at an optimal timing regardless of differences in lengths of transport objects in the transporting direction, and to an image processing apparatus equipped with the transport-object transporting device.
2. Related Art
As discussed in JP-A-2007-230657, image reading apparatuses, such as photocopiers, facsimile apparatuses, or scanners, equipped with auto document feeders, serving as an example of transport-object transporting devices, have already been developed. One example of an image reading apparatus of this type is an image reading apparatus that can continuously read image information recorded on both the front and back faces of a transport object (also referred to as “sheet” hereinafter). This image reading apparatus is configured to guide a sheet fed with its first face facing upward to a semi-loop-shaped transport path provided with a transport roller, an image reader, and a discharge roller, so as to transport the sheet with its first face facing downward and read an image from the first face. This operation of transporting a sheet with its first face as the subject face will be referred to as “first-face transportation” hereinafter.
Subsequently, the image reading apparatus switches the transporting direction so as to make the sheet enter an inversion path and guide the sheet again to the transport path. Thus, the image reading apparatus transports the sheet with its second face, opposite the first face, facing downward and reads an image from the second face. This operation of transporting a sheet with its second face as the subject face will be referred to as “second-face transportation” hereinafter.
The switching of the transporting direction of the sheet required when continuously reading the images from the first face and the second face of the sheet and the switching between the nip position and the release position of the discharge roller are performed at preset operation timings by a transport-object transporting device (also referred to as “sheet transporting device” hereinafter).
Sheet transporting devices of the aforementioned type are becoming smaller in size with size reduction of image reading apparatuses, resulting in shorter transport paths. Therefore, when transporting a sheet that is long in the transporting direction, apart of the sheet sent to the inversion path and another part of the sheet discharged outward from the transport path rub against each other at the nip point of the discharge roller during second-face transportation. For this reason, the discharge roller is set in the release position at a timing at which the rubbing occurs, thereby allowing for such rubbing.
However, when the switching of the transporting direction of the sheet and the switching between the nip position and the release position of the discharge roller are performed by using a single driving motor, the aforementioned operation timings therefor are fixed timings determined on the basis of a reduction ratio or the like of a gear train. For this reason, the length of sheets in the transporting direction that allows for continuous operation of first-face transportation and second-face transportation is limited to a certain range (between, for example, the longitudinal dimension of a letter-size sheet and the longitudinal dimension of an A4-size sheet). Therefore, in view of the problem related to the rubbing mentioned above, sheets with a length in a wide range of, for example, the lateral dimension of an A4-size sheet to a 17-inch-size sheet (i.e., 210 mm to 432 mm) cannot be handled in the sheet transporting device of the related art.
The switching between the nip position and the release position of the discharge roller can be performed using additional power that is independent of the power from the transport roller or the discharge roller by utilizing, for example, a solenoid disclosed in JP-A-2004-2024. However, this can lead to an increased number of components and to a complicated structure, resulting in an increase in the cost of the sheet transporting device.